Light managment

Hi,
I have a new question for this newsgroup about light management in
freshwater tank.
I have asked to my reseller how much light a tank will need. He answered me
that I can switch on the light in my tank when I am at home so I can enjoy
it. In fact, he said, in nature the amount of light that normally plants and
fishes receive from the Sun is very low so tank could work also with ambient
lights.
Actually I switch on the light (neon tube) a couple of hours in the morning,
to feed fishes, and about four-five hours in the evening. Everything seem to
be ok, but I would like to avoid that fishes become stressed.
So the question is how much and when should I turn on the light? Do I need a
special type of light?
Thank you very much....
Max from Rome